Breathable waders are great. However, rather than go through all of the gyrations of building some sort of sock layering process that may, or may not do the job, next time purchase a pair of breathables with a good thermal boot foot. It is the answer for winter fishing.
I purchased a pair of Hodgman bootfoot breatable waders before my winter in Wyoming (2000) and they were not only great for spring fishing (think of routinely breaking anchor ice to reach open water)but have been great now that I am back home in Washington. I just wear a poly liner sock, a pair of comfortable wool socks and slide into the waders. No water is too cold to penetrate the Hodgman bootfoot.
As for what to wear under your breathable waders, any cross-country skier or mountaineer will tell you that cotton is the enemy in our cold, wet outdoor winters. It holds moisture and draws heat from your body. Instead, pull on a pair of lightweight poly underwear over your skin then add a pair of heavier fleece and you'll be warm and dry.

Les Johnson